Fix WASM gallery startup stall/blank by forcing UnoRuntimeIdentifier

[MartinZikmund]

Problem

The WebAssembly gallery fails on startup: WinUI 2 / Uno.UI (production at toolkitlabs.dev) hangs on the splash screen at ~100% load, and WinUI 3 / Uno.WinUI shows a blank screen. No console error. Both have the same root cause and regressed with the net8→net9 migration that flipped the head SDK from Microsoft.NET.Sdk.Web to Microsoft.NET.Sdk.WebAssembly.

Root cause

1. The WASM head sets TargetFramework late — in App.Head.Wasm.props, imported in the project body.
2. NuGet's generated .nuget.g.props imports Uno.(UI|WinUI).Runtime.WebAssembly.props (which sets UnoRuntimeIdentifier=WebAssembly) inside an ImportGroup gated on '$(TargetFramework)' == 'net9.0'. That file is evaluated before the project body, when TargetFramework is still empty, so the import is skipped and UnoRuntimeIdentifier never gets set.
3. Uno's ReplaceUnoRuntime target is gated on UnoRuntimeIdentifier != '', so it never runs. That target swaps the platform-agnostic reference lib/net9.0/Uno.UI.dll (no embedded JS) for the uno-runtime/.../webassembly/Uno.UI.dll implementation that embeds Uno.UI.WasmScripts.Uno.UI.js.
4. So the JS-less reference assembly ships → Uno.UI.js is never delivered → globalThis.Uno.UI never registers → Application.Start's callback is never invoked → the splash never clears.

Diagnostic proof: dotnet build … --getProperty:UnoRuntimeIdentifier returns empty; adding -p:TargetFramework=net9.0 (early) returns WebAssembly.

Fix

Set UnoRuntimeIdentifier=WebAssembly and UnoIsWebAssemblyBrowserHead=true explicitly in App.Head.Wasm.props (what the skipped NuGet import would have set). The runtime-assembly swap runs late enough that a body PropertyGroup satisfies its gate.

Verification

Built and run in the browser for both flavors. After the fix: 11 MB runtime Uno.UI.dll deployed, Uno.UI.js present in the package, uno_dependencies includes Uno.UI, and the gallery renders fully (2386 XAML elements, loader gone) in both Uno.UI/WinUI 2 and Uno.WinUI/WinUI 3.

Note for maintainers

This is the minimal, low-risk fix. The deeper issue is that the late-TargetFramework pattern silently skips any net9.0-conditional NuGet import; longer-term, making TargetFramework known before .nuget.g.props (global property / real multi-targeting outer build) or migrating heads to Uno.Sdk would address the class of problem.

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